With federal agents testifying against their commanders, members of Congress calling for the top man's ouster and accusations that ATF is fudging its gun smuggling numbers, the political fallout itself has become fast and furious.

The operation, conducted jointly with agents from ATF, FBI, DEA and other agencies was aimed at reaching beyond the low-level "straw purchasers" of weapons and building a complex case against Mexican traffickers and their weapons brokers.

Firearms reach cartels

But the weapons purchased in gun stores in and around Phoenix, as many as 2,500, got away from ATF surveillance and eventually reached the cartels in Mexico. Two of them were recovered in December at the site in Southern Arizona where smugglers killed Border Patrol agent Brian Terry.

At his news conference Wednesday, President Barack Obama said letting guns go to Mexico "would not be an appropriate step by the ATF, and we've got to find out how that happened. ... As soon as the investigation is completed, appropriate actions will be taken."

ATF defenders, some of them retired agents, say the problem of guns from U.S. sources winding up in Mexico is a border-wide phenomenon, and Fast and Furious is just an operation gone wrong in a sea of other cases that ended in convictions.

"The problem of guns from the U.S. ending up in Mexico goes well beyond Fast and Furious, and it was there well before Fast and Furious got started," said Michael Bouchard, the ATF's assistant director for field operations from 2004 until his retirement in 2007.

The operation yielded an indictment in January that named 20 defendants, all low-level purchasers. The indictment identified purchases of 681 guns, including 589 AK-47s.

The political uproar is just the latest battle line in the long war over gun rights versus gun control. Issa and Grassley are widely considered folk heroes to gun-rights advocates who fear liberal Democrats using U.S. guns in Mexico as fodder for more firearms restrictions.

"We've been involved in this issue ever since various folks - the president of Mexico (Felipe Calderon) and President Obama - accused firearms dealers operating legally under the Second Amendment of being the source of violence in Mexico,“ said Andrew Arulanandam, spokesman for the National Rifle Association. "They're the ones advocating more gun control as a means of addressing the problem across the border."

Today, Democrats are sponsoring a forum focused on stopping the weapons flow through gun law "improvements."

Senators Dianne Feinstein, D-Calif., Charles Schumer, D-N.Y., and Sheldon Whitehouse, D-R.I., issued a report earlier this month, "Halting U.S. Firearms Trafficking to Mexico" that called for reinstatement of the assault weapons ban and an ATF-proposed requirement that firearms dealers report multiple purchases of assault-type weapons to the agency. They based the report on ATF data claiming 20,504 of 29,284 guns recovered in Mexico in 2009 and 2010 and submitted for tracing were U.S. sourced firearms.

That's 70 percent of the total 29,284 guns traced those years.

Ongoing investigations

According to ATF, 1,573 defendants faced charges related to firearms trafficking from 2006 to 2010 under Project Gunrunner, the ATF's five-year effort to combat weapons trafficking to Mexico. The agency has 4,600 on-going Gunrunner investigations in border jurisdictions, an ATF spokesman said.

Gun rights advocates who say ATF data is suspect point out that officials last year said 90 percent of guns traced from Mexico originated in the U.S., then downgraded it to 70 percent this year. Grassley wrote to ATF Acting Director Kenneth Melson challenging the 70 percent calculation.

"Unfortunately, this information paints a grossly inaccurate picture of the situation," he said, citing ATF figures he has showing only a quarter of weapons were traceable to the U.S.

A Hearst Newspapers survey last month of 44 gun prosecutions in Texas, Arizona, California, Nevada, Oklahoma and New Mexico pinpointed a total of 1,600 U.S.-purchased guns by brand name that were recovered in Mexico or intercepted en route.